A 35-year-old man from the Smethwick area of the West Midlands, who was among at least 30 people travelling on the coach, was pronounced dead at the scene.

Another male passenger and the driver of the lorry, which was returning to Devon from the Manchester area, both suffered life-threatening injuries and were taken to the trauma unit at Birmingham's Queen Elizabeth Hospital.

Witnesses and a survivor of the crash said the accident happened after the coach came to a halt close to a footbridge in Bartley Green, Birmingham.

The driver's side of the lorry's cab was left embedded in the rear nearside of the coach following the collision, which also left debris strewn across the approach to the footbridge.

A West Midlands Police spokesman said: "A 49-year-old bus driver from Birmingham has been arrested on suspicion of causing death by dangerous driving following a dawn crash on the M5 this morning.

"The arrested man has been released on police bail pending further enquiries."

The single-decker bus was not fitted with seatbelts and is thought to have been carrying fruit-pickers en route from Birmingham to the Evesham area of Worcestershire.

Signs warning road users of fog in the area had been active since 4.46am but police declined to speculate whether low visibility was a contributory factor in the collision.

Chief Inspector Carl Flynn, of the Central Motorway Police Group, said: "This was a very serious collision which has tragically claimed one life.

"Emergency service personnel worked together in very difficult conditions to assist survivors and to ensure the safety of other road users.

"A thorough police investigation into the cause of the collision is now under way. This is likely to take some time and, at this stage, we will not speculate on the causes of the collision.

"I would like to extend my sympathies to the family and friends of the deceased, and our thoughts remain with the survivors and their loved ones."

Motorists faced lengthy queues in the immediate area as traffic police closed off the southbound carriageway to examine the accident scene, near to Frankley Services, and drafted in a police helicopter, apparently to take aerial photographs of the site.

Chief Inspector Flynn said a 999 call had been made alerting the authorities to the stranded coach minutes before the crash was reported.

Addressing a news conference at a Highways Agency control centre in Quinton, Birmingham, the officer told reporters: "At 6.12am a 999 call was made by a concerned member of the public about a coach having broken down on lane one of the M5 south.

"The Highways Agency did the appropriate signage on the network and dispatched their resources to the scene of the broken-down coach as is normal practice.

"However, unfortunately at 6.24am a further call was received to say that a large goods vehicle had collided into the rear of the coach.

"Central Motorway police officers were immediately dispatched and arrived at the scene six minutes later and then assisted colleagues from the Highways Agency, the ambulance service and fire service to manage the scene."

It is estimated that up to 40 people may have been on board the bus when the crash occurred and casualties were also taken to Birmingham's City Hospital, Alexandra Hospital in Redditch and Sandwell General Hospital.

Martin Stott, of the Highways Agency, told the press conference that signs warning of an incident and urging drivers to slow down were put in place after the initial call saying the coach had broken down.

Fog warnings were already in place, Mr Stott said, adding: "We'd set information across the network on our display signs to say that there was fog and to slow down."

Anyone who is concerned for the welfare of a relative can contact a casualty bureau set up by West Midlands Police on 0800 0920410.